1. Field of the Invention
This invention relates to the use of the latent heat of fusion of a substance to store a relatively large quantity of isothermal heat energy very near a heating tool's point of contact with a workpiece, thereby stabilizing the temperature of the tool while it heats the work. More specifically, it relates to replaceable soldering iron tips that contain a fusible substance within a porous metal matrix.
2. Description of the Prior Art
A manual soldering or desoldering operation commonly consists of bringing a heated tool into contact with one or more of the surfaces to be joined or separated, holding this contact until enough heat has been delivered to the workpiece(s) to raise the surface temperature(s) thereof above the melting point of an appropriate solder alloy, and then applying (or removing) said alloy to (or from) these surfaces.
The heated tool typically idles at an equilibrium temperature established by the balance between the power input to the heating element and the heat dissipated from the apparatus due to free air convection. More sophisticated apparatus will employ the output of a temperature sensing transducer associated with the heated tool to control the power generated by the heating element. In any event, the temperature at the working surface of the heated tool drops rapidly when it is brought into contact with the workpiece. The workpiece, being initially at a much lower temperature, acts as a sink for the heat stored in the heated tool. The replenishment of this stored heat is through a relatively long and complex conductive path from the heating element, and typically involves time lags and temperature drops that are not only immoderately large, but are also ill-defined and poorly controlled.
For the circumstances most germane to this invention, the heated tool is a replaceable soldering iron tip, the surfaces to be joined or separated are parts of an electric or electronic circuit, and the primary function of the joint is to complete an electrically conductive junction between circuit elements. Since an electric or electronic apparatus can be no more reliable than its interconnection network, the fact that manually made solder joints are among the most frequent points of failure has let to the most intense scrutiny of the conditions under which they are formed. The uncertainties stemming from the poorly controlled time lags and temperature drops noted above are intensified by the typical hand soldering process wherein a series of solder joints are made in rapid succession. The progressive drop in soldering iron tip temperature must necessarily render the formation conditions of each solder joint different from the next. A brief treatment of these progressive temperature drops is attempted on page 293 of Howard Manko's definitive book Solders and Soldering, second edition, but its very brevity points up the neglect of this important issue, and even the illustrative figure is seriously flawed. The only prior art known to the Inventor that addresses this situation is the simple and obvious expedient of increasing the thermal mass of the soldering iron tip by increasing its size, an approach typified by the "Power Boost" line of tips offered by Hexacon Electric Company of Roselle Park, N.J. This use of the specific heat capacity of the tip material to store and release heat energy is effective only to the extent that it reduces, in rough proportion to the increase in mass of the tip, the temperature drops resulting from a given operation.
The ideal soldering iron tip would maintain a fixed temperature at the point of contact with the workpiece while delivering an unlimited amount of heat at a rate determined only by the physical specifics of the workpiece.
It is a primary object of this invention to provide a replaceable soldering iron tip having an unusually high heat capacity at a stable temperature, thereby providing the means to restrain the temperature excursions, including the progressive temperature drops described above, that contribute to solder joint variations.